1. Field of the Invention
The present invention relates a method of manufacturing a semiconductor device having a compound semiconductor layer made of a group III-V nitride-based semiconductor (hereinafter referred to as nitride-based semiconductor) such as GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), or TlN (thallium nitride) or mixed crystal thereof, and a method of forming a nitride-based semiconductor layer.
2. Description of the Prior Art
In recent years, GaN-based semiconductor light emitting devices have been developed for commercial use as a light emitting diode emitting blue or violet light or a semiconductor light emitting device such as a semiconductor laser device.
In the manufacture of the GaN-based semiconductor light emitting device, a GaN-based semiconductor layer is formed by epitaxy growth on an insulator substrate such as a substrate of sapphire (Al2O3), since there is no substrate made of GaN.
In this case, GaN and sapphire have different lattice constants, and the GaN-based semiconductor layer cannot be grown at a high temperature directly on the sapphire substrate. Therefore, when a GaN-based semiconductor layer is to be grown on a sapphire substrate, a low temperature buffer layer made of GaN or AlN in an amorphous state is grown on the sapphire substrate at a substrate temperature near the range of 500xc2x0 C. to 600xc2x0 C., and then a GaN-based semiconductor layer is grown at a high temperature near 1000xc2x0 C. on the low temperature buffer layer. Thus, the GaN-based semiconductor layer can be grown on the sapphire substrate.
In the disclosure of Japanese Patent Publication No. 8-8217, for example, GaN is grown for one minute on a sapphire substrate at a substrate temperature of 500xc2x0 C., and a GaN low temperature buffer layer having a film thickness of 200 xc3x85 is formed. According to Japanese Patent No. 3026087, AlN is grown for two minutes on a sapphire substrate at a substrate temperature of 650xc2x0 C., and an AlN low temperature buffer layer having a film thickness of 300 xc3x85 is formed.
Meanwhile, the low temperature buffer layer has a film thickness as small as in the range from 200 xc3x85 to 500 xc3x85, the growth rate is lowered so that the film thickness can readily be controlled at the time of growing the low temperature buffer layer. In the above Japanese Patent Publication No. 8-8217, for example, a GaN low temperature buffer layer is grown at a growth rate of 3.33 xc3x85/sec, while according to Japanese Patent No.3026087, the growth rate is 2.5 xc3x85/sec. Note that the growth rate of the low temperature buffer layer is adjusted by the supply amount of a material gas such as gallium and aluminum.
In the growth of the low temperature buffer layer, the low temperature buffer layer is typically grown at such a low growth rate as the above. Note that the effect of the growth rate of the low temperature buffer layer upon the crystallinity of the GaN-based semiconductor layer has not been examined, and the low temperature buffer layer is typically grown at the low growth rate as the above.
When a GaN-based semiconductor layer is grown on a low temperature buffer layer which has been grown under the optimum growth conditions, more specifically at the optimum growth temperature and into the optimum film thickness, good crystallinity and good electrical characteristics are achieved in the GaN-based semiconductor layer. Meanwhile, a GaN-based semiconductor layer grown on a low temperature buffer layer which has been grown under conditions departed from the optimum conditions does not have good crystallinity and good electrical characteristics. As a result, in order to manufacture a semiconductor light emitting device having good device characteristics and high reliability, a low temperature buffer layer must be grown under the optimum growth conditions and then a GaN-based semiconductor layer must be grown on the buffer layer.
However, the range of the optimum growth conditions for such a low temperature buffer layer would tend to be very small. The range of the optimum growth conditions would be particularly narrow for the lower temperature buffer layer made of GaN.
Therefore, when a GaN-based semiconductor layer is grown on a sapphire substrate in a new crystal growth system, the optimum growth conditions for the low temperature buffer layer must be specified. Much labor and time should be necessary for specifying the optimum growth conditions for such a low temperature buffer layer.
If the optimum growth conditions are specified for the low temperature buffer layer and the crystal growth system is set so that the conditions are satisfied, the conditions under which the low temperature buffer layer grows sometimes depart from the range of the optimum growth conditions as the condition of the crystal growth system changes. As a result, a GaN-based semiconductor layer having good crystallinity and electrical characteristics can hardly be stably provided with high reproducibility.
For example, the repetition of a crystal growth process for a long period of time in a crystal growth system causes byproducts by crystal growth to be obtained, and the byproducts accumulated in the reaction tube of the crystal growth system change the condition of the reaction tube. The change in turn causes the conditions in the growth of the low temperature buffer layer to depart from the range of the optimum growth conditions during growth. Therefore, a high quality, GaN-based semiconductor layer cannot stably be produced with good reproducibility.
It is an object of the present invention to provide a method of forming a nitride-based semiconductor layer allowing a high quality, nitride-based semiconductor layer to be stably provided with good reproducibility.
Another object of the present invention is to provide a method of manufacturing a nitride-based semiconductor device allowing a nitride-based semiconductor device having a high quality, nitride-based semiconductor layer and good device characteristics and high reliability to be stably provided with good reproducibility.
A method of forming a nitride-based semiconductor layer according to one aspect of the present invention includes the steps of growing a buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) on a substrate at a growth rate of at least 7 xc3x85/sec, and growing a nitride-based semiconductor layer of AlaBbIncTldGa1xe2x88x92axe2x88x92bxe2x88x92cxe2x88x92dN(0xe2x89xa6a less than 1, 0xe2x89xa6b less than 1, 0xe2x89xa6c less than 1, 0xe2x89xa6d less than 1, a+b+c+d less than 1) on the buffer layer.
According to the method of forming a nitride-based semiconductor layer, the buffer layer is grown at a high growth rate, and therefore a good buffer layer can stably be provided with good reproducibility regardless of changes in the condition of a crystal growth system. Therefore, a nitride-based semiconductor layer is grown on such a buffer layer, so that a nitride-based semiconductor layer having good crystallinity and good electrical characteristics can stably be provided with good reproducibility if there is a change in the condition of the crystal growth system.
The buffer layer is preferably grown at a growth rate of at most 51 xc3x85/sec. Thus, a good buffer layer can stably be provided with good reproducibility regardless of changes in the condition of the crystal growth system, and the film thickness of the buffer layer can readily be controlled.
The buffer layer is more preferably grown at a growth rate in the range from 16 xc3x85/sec to 42 xc3x85/sec. The growth of the buffer layer at the growth rate allows a good buffer layer to be stably provided with good reproducibility. Thus, a nitride-based semiconductor layer having better cyrstallinity and electrical characteristics can stably be provided with good reproducibility.
The buffer layer is more preferably grown at a growth rate in the range from 25 xc3x85/sec to 29 xc3x85/sec. The growth of the buffer layer at the growth rate allows an even better buffer layer to be stably provided with good reproducibility. Thus, a nitride-based semiconductor layer having even better crystallinity and electrical characteristics can stably be provided with good reproducibility.
The growth rate of the buffer layer may be adjusted by adjusting the supply amount of a group III element supplied at the time of growing the buffer layer. Thus, the growth rate of the buffer layer can readily be controlled.
The step of growing the buffer layer more preferably includes growing the buffer layer to have a film thickness in the range from 50 xc3x85 to 300 xc3x85, and more preferably to have a film thickness in the range from 100 xc3x85 to 200 xc3x85.
The step of growing the buffer layer preferably includes growing the buffer layer at a substrate temperature in the range from 500xc2x0 C. to 700xc2x0 C. and more preferably at a substrate temperature in the range from 550xc2x0 C. to 650xc2x0 C.
The step of growing the nitride-based semiconductor layer may include forming as the active device region a light emitting layer or an active layer in a semiconductor light emitting device, a core layer in a waveguide device, an I layer in a PIN photodiode, a pn junction portion in a photodiode or a hetero-junction bipolar transistor or a channel portion in an field effect transistor.
The step of growing the nitride-based semiconductor layer may include forming a cladding layer of a first conductivity type, an active layer and a cladding layer of a second conductivity type in this order.
A method of manufacturing a nitride-based semiconductor device according to another aspect of the present invention includes the steps of growing a buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) on a substrate at a growth rate of at least 7 xc3x85/sec, and growing a nitride-based semiconductor layer including an active device region on the buffer layer and made of AlaBbIncTldGalxe2x88x92axe2x88x92bxe2x88x92cxe2x88x92d N(0xe2x89xa6a less than 1, 0xe2x89xa6b less than 1, 0xe2x89xa6c less than 1, 0xe2x89xa6d less than 1, a+b+c+d less than 1) on the buffer layer.
Note that the active device region in the nitride-based semiconductor device in this case corresponds to a light emitting layer or an active layer in a light emitting diode device or a semiconductor laser device, a core layer in a waveguide device, an I layer in a PIN photodiode, a pn junction portion in a photodiode or an HBT (hetero-junction bipolar transistor) or a channel portion in an FET (field effect transistor).
In the method of manufacturing a nitride-based semiconductor device, the buffer layer is grown at a high growth rate, and therefore a good buffer layer can stably be provided with good reproducibility regardless of changes in the condition of a crystal growth system. As a result, by growing a nitride-based semiconductor layer including an active device region on the buffer layer, a nitride-based semiconductor layer having good crystallinity and electrical characteristics can stably be provided with good reproducibility when there is a change in the condition of the crystal growth system. Thus, a nitride-based semiconductor device having good device characteristics and high reliability can stably be provided with good reproducibility.
The buffer layer is preferably grown at a growth rate of at most 51 xc3x85/sec. Thus, a good buffer layer can stably be provided regardless of changes in the condition of a crystal growth system, and the film thickness of the buffer layer can readily be controlled.
The buffer layer is preferably grown at a growth rate in the range from 16 xc3x85/sec to 42 xc3x85/sec. The growth of the buffer layer at the growth rate allows a better buffer layer to be stably provided with good reproducibility. Thus, a nitride-based semiconductor device having better crystallinity and electrical characteristics can stably be provided with good reproducibility.
Furthermore, the buffer layer is preferably grown at a growth rate in the range from 25 xc3x85/sec to 29 xc3x85/sec. The growth of the buffer layer at the growth rate allows an even better buffer layer to be stably provided with good reproducibility. Thus, a nitride-based semiconductor device having better crystallinity and electrical characteristics can stably be provided with good reproducibility.
The step of growing the buffer layer may include adjusting the growth rate of the buffer layer by adjusting the supply amount of a group III element supplied at the time of growing the buffer layer. Thus, the growth rate of the buffer layer can readily be controlled.
The step of growing the buffer layer preferably includes growing the buffer layer to have a film thickness in the range from 50 xc3x85 to 300 xc3x85 and more preferably includes the step of growing the buffer layer to have a film thickness in the range from 100 xc3x85 to 200 xc3x85.
The step of growing the buffer layer more preferably includes growing the low temperature buffer layer at a substrate temperature in the range from 500xc2x0 C. to 700xc2x0 C., and more preferably in the range from 550xc2x0 C. to 650xc2x0 C.
The step of growing the nitride-based semiconductor layer may include forming as the active device region a light emitting layer or an active layer in a semiconductor light emitting device, a core layer in a waveguide device, an I layer in a PIN photodiode, a pn junction portion in a photodiode or a hetero-junction bipolar transistor or a channel portion in an field effect transistor.
The step of growing the nitride-based semiconductor layer may include forming a cladding layer of a first conductivity type, an active layer and a cladding layer of a second conductivity type in this order.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.